1. Field of the Invention
The present invention relates to a semiconductor device in which pluralities of semiconductor chips are stacked on a substrate.
2. Description of Related Art
In recent years, a structure in which semiconductor chips are stacked within a package is required for a semiconductor device which comprises semiconductor chips, in order to realize a high density array of semiconductor chips, associated with diversified functions. In a semiconductor device of such a structure, semiconductor chips are generally connected to a substrate through wire bonding. However, when the wire bonding is used as wires for connecting respective semiconductor chips to a substrate, long wires required therefore will prevent fast operation of the semiconductor chips. Accordingly, a semiconductor device with reduced lengths of wires has been proposed and disclosed in Japanese Patent Laid-Open No. 2004-281980. Japanese Patent Laid-Open No. 2004-281980 describes a semiconductor device in which a semiconductor chip is formed with through electrodes such that the semiconductor chip and substrate are interconnected through the through electrodes, in order to reduce the length of wires.
Also, in some semiconductor devices structured to stack semiconductor chips within a package, a plurality of substrates, each mounted with a semiconductor chip, are stacked, and the substrates are interconnected with bumps. In the semiconductor device of this structure, the semiconductor chips are generally mounted on the substrates by heating. At this time, due to a difference in the thermal coefficient of expansion between silicon which is a main component of the semiconductor chips and a resin which is a main component of the substrates, the degree of bowing differs depending on the type of materials. Consequently, the bumps undergo peel stress which acts to peel off the connections between the substrates. Thus, a semiconductor device for alleviating this peel stress has been proposed and disclosed in Japanese Patent Laid-Open No. 2005-268299. Japanese Patent Laid-Open No. JP-2005-268299 describes a semiconductor device. The semiconductor device is provided with a particular region where bumps are arrayed on only one end side of a substrate on which no semiconductor chip is mounted, in order to alleviate a peel stress.
In a semiconductor device which comprises a plurality of semiconductor chips formed with through electrodes, mounted on a substrate, like the semiconductor device described in Japanese Patent Laid-Open No. 2004-281980, an electronic circuit provided for each semiconductor chip is generally formed on a flat planar base made of silicon nitride or polyimide, which has a thermal coefficient of expansion larger than that of silicon which is a main component of the semiconductor chip, in a high-temperature atmosphere. For this reason, at room temperatures, the circuit formation surface largely contracts as compared with silicon, resulting in bowing of the semiconductor chip. Now, a description will be given of a problem which can arise when two semiconductor chips connected to each other and stacked one on the other differ in foot print (chip size) and/or materials used for the circuit formation surface.
FIG. 1 is a cross-sectional view of an exemplary state showing how each chip bows when two semiconductor chips are stacked on a substrate and connected to each other, where they differ from each other in foot print and materials used for the circuit formation surface. In FIG. 1, logic chip 101 and memory chip 111 which has a larger foot print than logic chip 101, are stacked on substrate 121. A logic circuit is arranged on circuit formation surface 102 of logic chip 101 which is formed without using polyimide. On the other hand, a memory circuit is arranged on circuit formation surface 112 of memory chip 111 which is formed using polyimide. Also, conduction bumps 201 are arranged between through electrodes 113 which extend through memory chip 111 and through electrodes 103 which extend through logic chip 101, such that through electrodes 113 and through electrodes 103 are connected by these conduction bumps 201.
Since logic chip 101 and memory chip 111 differ in foot print and materials used to form the circuits, the amount in which logic chip 101 will bow, as a whole, is smaller than the amount in which memory chip 101 will bow. This difference in the amount of bowing is larger at points further away from the center of the chips, as shown in FIG. 1. Since a larger difference in the amount of bowing results in an increase in the peel stress that acts on conduction bumps 201 to peel off connections between the chips, conduction bumps 201 are more susceptible to rupture since they are located closer to the outer periphery of logic chip 101. Consequently, a defective connection is more likely to occur between logic chip 101 and memory chip 111.
In regard to the above problem, the semiconductor device disclosed in Japanese Patent Laid-Open No. 2005-268299 is provided on the precondition that a plurality of substrates, each mounted with a semiconductor chip, are stacked one on another, and comprises bumps arrayed on one end side in regions of the substrates on which chips are not mounted. Accordingly, JP-2005-268299A is not applicable to a semiconductor device which comprises a plurality of semiconductor chips formed with through electrodes, which are stacked one on anther and connected through conduction bumps, as shown in FIG. 1.